CHAPTER XVI. 



UNSEGMENTED WORMS. 



204. The forms described in this chapter do not belong 

 to a single branch or phylum of the animal kingdom. 

 Most zoologists regard them as sufficiently different in 

 form and structure to make at least four branches. 

 The species are not very numerous and the individuals 

 are not large. 



To the zoologist these small groups are interesting 

 because : 



1. They are the lowest of the many-celled animals 

 that show a clear development of a long axis with anterior 

 and posterior ends, right and left sides, and dorsal and 

 ventral surfaces (bilateral symmetry) . 



2. The various divisions show, especially in their 

 embryonic development, certain similarities to the well- 

 recognized branches of animals; so it may be that some 

 of the higher phyla sprang originally from types like 

 these. 



3. These groups have not been so thoroughly studied 

 in the past as some of the more developed animals. 



To the elementary student they are of interest because 



of their wide-spread occurrence on the face of the earth 



in all sorts of conditions and places; because so many of 



the harmful and dangerous parasites on man and the 



1 86 



